1. Field of the Invention
The invention relates to memory control, and in particular to memory controllers providing alternative pad sequences and pinout sequences.
2. Description of the Related Art
Memory is essential in most electronic applications, generally requiring not only high capacity but also high data transfer rate. One type of DRAM is DDR (Double Data Rate), providing increased bandwidth over preceding single-data-rate SDRAM (synchronous dynamic random access memory) by transferring data on both the rising and falling edges of a clock signal.
DDR (DDR1) is superseded by DDR2, implementing modifications to allow higher clock frequency, but operating on the same principle as DDR1. DDR2 has become a logical progression for memory standards and speeds, incorporating several new designs and specifications which play a part in increased speed. For example, DDR2 requires on-die termination (ODT) to eliminate excess signal noise while DDR1 requires only on-board termination. DDR2 and DDR1 use different external voltages (2.5 V and 1.8 V). DDR2 requires off-chip driver (OCD) impedance calibration while DDR1 does not. DDR2 uses a 4-bit prefetch while DDR1 uses a 2-bit prefetch. DDR2, using a Fine Ball Grid Array (FBGA), can be made smaller then DDR1 which uses Thin Small Outline Package (TSOP).
DDR2 and DDR1 are currently in a transitional stage in the field of electronic applications. Some cost-sensitive electronic applications may prefer DDR1 to DDR2, even though DDR2 is superior in performance. To support evolving requirements, the two designs and specifications are often combined in a single chip such that timely development and supply of various types of DRAMs or memory controllers can be achieved. A circuit supporting DDR1 and DDR2 on the same chip through interconnection layer switching has been introduced. Switching to accommodate different standardized on-chip designs and specifications for DDR1 and DDR2 has been implemented in the DDR1/DDR2 mixed chip. Nevertheless, satisfactory support of both DDR1 and DDR2 entails more than standardized on-chip designs and specifications.